An undesirable result in the manufacture of transparent films, such as optical films, is the occurrence of either functional or cosmetic defects in the film. There are numerous potential causes of these defects and they generally manifest themselves in different manners. For example, for transparent and semi-transparent films, surface scratches may scatter light, caliper variations may cause slight refractive variations, and debris may block transmitted light or scatter reflective light. Because of the subtle nature of defects, the small size of the defects, and variety of defects, it is difficult to manually inspect outgoing film to ensure quality.
Various attempts have been made to automatically inspect optical films, thereby improving defect detection capabilities and reducing costs when compared to manual inspection. While some have been successful, one difficult issue has been the inability of automated systems to detect caliper change defects that manifest themselves as subtle flow patterns in the film. Further, the detection of different types of defects generally requires more than a single optical configuration. For example, to detect scratches, flow patterns, and debris may require two or three optical configurations along with associated processing electronics.
Conventionally practiced web inspection systems generally incorporate line scan cameras that are capable of detecting scattering defects such as scratches or surface particles. The line scan cameras are configured with dark-field optics in either reflected or transmitted mode and are capable of detecting the noted defects. However, this single configuration is not sufficient for detecting refracting defects or embedded particles that do not distort the surface of film.
Laser scanners can be configured with multiple channels in order to be sensitive to all defects at the same time. Basically, this is the same as having multiple systems, one for each channel, which increases the system complexity and cost. Also, laser scanners are large, complex electromechanical systems that are expensive to implement in production environments. Laser scanners are also difficult to run in parallel for inspection applications requiring extremely high resolution across wide webs.
Another imaging technique is referred to as schlieren imaging. This technique is generally useful for measuring variations in the refractive index within an optical system. The technique detects inhomogeneities within a medium by detecting the energy refracted by that portion of the medium in which the inhomogeneity occurs. Schlieren imaging is generally used over long distances with large objects. The system is not suitable for film inspection because it is only capable if detecting changes in the refractive index of the medium.
It would be advantageous to provide a single imaging method and device capable of detecting and quantifying different optical properties of transparent films. It would also be an advantage to detect refracting, obstructing and scattering defects with a single imaging configuration.